The present invention relates to an electron microscope for the observation of point defects, impure atoms and their clusters which exist at joint interfaces and contacts in an integrated device formed into a layered structure such as a memory or fast-calculation device.
The present invention also relates to an electron detection instrument for an electron microscope, especially for the purpose described above. More particularly, it concerns an electron detection instrument for observing an electron microscope image corresponding to a specific atomic configuration or crystal structure of a specimen by way of measuring scattered, diffracted, refracted, or transmitted electrons through a specimen at a high sensitivity and a high signal-to-noise ratio in a desired range of detection angle.
As described in Proc. Mat. Res. Soc. Symp. Vol. 183 (Materials Research Society, San Francisco, 1990) p. 55, the conventional electron microscope can be used for inferring a 3-dimensional atomic arrangement from several electron microscope images observed from different directions. In addition, a technique for obtaining a 2-dimensional image of a 3-dimensional atomic structure is disclosed in Japanese Patent Laid-open No. 61-78041.
On the other hand, a prior electron detection instrument for an electron microscope is constructed as shown in FIG. 18, disclosed in the "Ultramicroscopy," 28, (1989), 240. In the figure, the electron detection instrument is placed in an STEM (scanning transmission electron microscope). The STEM comprises an electron gun 20, illumination lenses 21, electron deflector coils 22, and objective lenses 23. The electron detection instrument comprises an electron-photon converting scintillator 31, light guides 32, photomultipliers 33, and a monitor 34. There are provided two separate electron detection instruments for use of different detected electrons. One electron detection instrument is for observing a dark-field image only with electrons scattered at a high angle, having an annular scintillator 31a. The other electron detection instrument is for observing a bright-field image only with transmitted electrons at a low angle, having a circular scintillator 31b. In operation, the scintillator 31 detects electrons from the specimen, and then converts these electrons to photons. These photons are fed to the photomultiplier. A signal output of the photomultiplier 33 corresponds to intensity of the electrons. The output signal is synchronized with scanning the incident electron beam by a scanning circuit 25 before being brightness-modulated and displayed on the monitor 34. The monitor 34 shows an electron microscope image.
A prior imaging instrument for electron microscope is constructed as shown in FIG. 19, the instrument being described in the "Instruction Manual Model 622SC Fiber Optically Coupled TV System," 1991, Gatan Inc., 6678 Owens, Dr, Pleasanton, Calif. 94588. To pick up an electron microscope image, the imaging instrument is placed on a flange provided at a bottom of a camera chamber 28 of the electron microscope. The imaging instrument comprises an electron-photon converting scintillator 41, optical fiber plates 42, an image intensifier 43, a prior photoconduction-type imaging device 44, an imaging device control system 45, and a monitor 34. One optical fiber plate 42 is placed between the scintillator 41 and image intensifier 43 and the other optical fiber plate 42 between the image intensifier 43 and the photoconduction-type imaging device 44, the both being faced with each other to couple. The prior photoconduction-type imaging device 44 was a imaging tube (representative trade name: Newbicon) having Zn.sub.1-x Cd.sub.x Te used for a photoconduction face thereof or a imaging tube (representative trade name: SIT tube) having Si used for a photoconduction face thereof. The quantum effect of both imaging tubes 44 become maximum at a light wavelength of 500 to 750 nm. To make a highly sensitive imaging, the scintillator 41 used for converting an electron image to optical image was the one disclosed in, for example, "Electron Microscopy," Japanese Society of Electron Microscopy, Vol. 27, No. 2, p. 170(1992). The scintillator 1 has a YAG (Y.sub.3-x Ge.sub.x Al.sub.5 O.sub.12) of 550 nm peak luminescence wavelength doped with cerium or GOS (Gd.sub.2 O.sub.2 S) of 510 nm peak luminescence wavelength doped with praseodymium, cerium, or fluorine.
In FIG. 19, electrons transmitted through specimen (not shown) pass through electron lenses 26, and form an electron microscope image on a fluorescent plate 27, whenever a fluorescent plate 27 closes an opening separating a column 29 and a camera chamber. In operation of the imaging apparatus, at first a fluorescent plate 27 is drawn up from the opening so as to project an electron microscope image onto the scintillator 41.
The scintillator 41 converts electrons to photons. The converted photons are in proportion to intensity of the electron microscope image, or number of the electrons per area. The photons pass the optical fiber plate 42 before coming to the image intensifier 43. The image intensifier 43 converts photons to electrons to magnify more than 100 times before converting the electrons to photons again. The magnified photons pass the optical fiber plate 42 to the photoconductive-film of the photon reception surface of the photoconduction-type imaging device 44 to emit electron-hole pairs. The generated current is detected by an electron beam emitted from an electron gun of the photoconduction-type imaging device 44 to obtain an output signal. The electron beam is scanned at a TV rate of 1/30 sec per screen. As a result, the electron microscope image on the photoconductive-film can be picked up in the same way as an ordinary TV camera.
With the conventional techniques mentioned above, it is necessary to prepare a large number of thinned pieces having a thickness of the order of several nm by cutting a specimen in various directions. In this case, if a target structure in the specimen has an infinitesimal size of the order of nanometers, it is impossible to cut the structure into a plurality of pieces and, thus, impossible to carry out 3-dimensional observation. Even if the target structure is large enough to allow the thinned pieces to be prepared, only part of the target structure is contained in such a piece so that a lot of information is found missing when constructing a 3-dimensional structure based on the electron microscope images of the pieces. In addition, since the observer has to infer a 3-dimensional structure while taking the relation between observation directions and their electron microscope images of thinned pieces, the technique results in very inadequate precision. The accuracy of the observation directions is effected by errors in the angle setting when specimen pieces are cut out and inclinations of the specimen pieces set on the specimen holder of the electron microscope. It is difficult to make the observation conditions by electron microscopes completely uniform for all the specimen pieces. The resulting errors thus give rise to variations in image contrast. An inference image formed by diffracted electrons, or a lattice image, varies depending upon, among other things, the thickness of the specimen and electron diffraction conditions. In addition, even though information on the atomic arrangement can be obtained from a lattice image, it is difficult to identify the atomic species of impurities and point defects.
In addition, it is disclosed in Japanese Pat. Laid-open No. 61-78041 that the electron incidence direction to the specimen surface is fixed and all reflected characteristic X-rays generated in the specimen can be obtained by changing the direction of detection. Information on the structure of a 3-dimensional atomic arrangement close to the surface is thereby obtained. Nevertheless, the obtained information is limited to one to two atomic layers on the surface due to the use of all the reflected characteristic X-rays. In addition, since the characteristic X-rays are generated from a region of the micron order, it is impossible to obtain high resolution at an atomic level. It is thus extremely difficult to obtain a 3-dimensional atomic arrangement in the bulk with a high resolution at an atomic level.
On the other hand, the prior electron detection instrument described above has the fixed shape of the detector. For the annular detector shown in FIG. 18, for example, an angle range of electron detection of q1 to q2 is determined in terms of a distance (camera length) between the specimen 24 and the scintillator 31. If an enlarging electron lens is placed between the specimen and the detector, the camera length can be varied. However, the angle range of electron detection changes in proportion to the camera length only. It is therefore impossible to set the angle range of electron detection to a desired one. The sole solution is to take the method that a multiple of detectors having different angle ranges of electron detection are prepared and are replaced depending on an observation object. This solution is expensive and takes too much time and labor for replacement and adjustment to use for practical work.
The prior imaging instrument also has the detectors (e.g. scintillators 31) for transmitted electrons and diffraction pattern provided separately from the one for high-angle scattered electrons. This is disadvantageous in that the detectors can serve only for the respective uses. In particular, the detectors for diffracted pattern must have such a multiple of pixels as a CCD (charge-coupled device) camera. The detector cannot be used in common with the prior ones for transmitted electrons and high angle scattered electrons. This means that a user must unavoidably prepare a plurality of exclusive detectors.
The prior imaging instrument for electron microscope (shown in FIG. 19) has the pixels needed to measure the image. The imaging instrument therefore can detect the scattered, diffracted, refracted, or transmitted electrons through a wide range of angle. However, the electron beam emitted from the electron gun of the imaging device 44 (situated bottom of the imaging device, not shown) is usually scanned in a square area on the surface lying along the lower optical fiber plate 42. The imaging instrument is not used as in the annular detector of the electron detection instrument. The detector therefore cannot detect the electron beam in a desired detection angle range. Also, the prior photoconduction-type imaging device 44 used in the imaging section is too low in sensitivity. Therefore, the image intensifier 43 is necessary to magnify the image intensity. The image intensifier 43, however, generate great amount of quantum noises through its magnification process. This results in terribly bad image quality. Such a phenomenon also occurs in the prior above-described electron detection instrument (shown in FIG. 18) since the photons are magnified by the photomultiplier 33. In particular, the electron detection instrument cannot detect the high-angle scattered electron beam at high signal-to-noise ratio since the beam is too weak. Further, if the scintillator 41 is used for the imaging device 44 having a maximum photon reception sensitivity outside the luminescence wavelength of 500 to 700 nm of the scintillator 41, the sensitivity is lowered.
The scattering angle distribution of the scattered electron intensity emitted from a specimen depends on the atomic number Z of the specimen component element. When a specimen comprises two component elements of high Z and low Z, if the scattering angle of scattered electrons detected by an electron detector is set to .alpha. to .beta., an electron microscope image in which both elements are contrast-emphasized as an intensity difference of the hatched part can be observed. From comparison of quantitative determination of contrast and contrast calculation using the electron scattering theory, the atomic number Z can be determined, that is, the atomic species can be determined. To execute such an observation, it is necessary that the scattering angle range of scattered electrons detected by the electron detector can be set optionally. When a dark-field detector of a conventional scanning transmission electron microscope is used, the scattering angle range of scattered electrons detected is determined uniquely by the distance (camera length) between a specimen and the detection surface and the inner and outer diameters of an annular scintillator. Therefore, it is impossible to set the scattering angle range of scattered electrons optionally.
According to the prior art, it is described that 3-dimensional coordinates are identified on the basis of a plurality of 2-dimensional images observed in various directions and a 3-dimensional image is reconstructed. However, an actual means for realizing reconstruction of a 3-dimensional image and an artifact reduction method for restructure are not described.
According to the filter correction back projection method which is a conventional 3-dimensional restructure method, if a projection image of a specimen in all directions cannot be obtained in principle, the 3-dimensional structure cannot be reconstructed. However, a projection image of a specimen in all directions cannot be observed by a transmission electron microscope due to restrictions on the equipment constitution thereof and the specimen shape. Therefore, when the 3-dimensional structure of a specimen is reconstructed from a transmission electron microscope image using the conventional 3-dimensional reconstructure method, a problem arises that remarkable artifacts are generated.